US2319052A - High speed method of making cellulose organic derivative film and sheeting - Google Patents

Description

May 11, 1943. c. R. FORDYCE z-rm. 2,319,052

HIGH SPEED METHOD OF MAKING CELLULOSE ORGANIC DERIVATIVE FILM AND SHEETING Filed Dec. 10, 1938 5 Sheets-Sheet 1 28 o o c CHARLES R. F ORDITE WALKER F. HUNTER, JR.

INVENTORS ATTO 1 May 11, 1943.

v C. R. FORDYCE ET AL HIGH SPEED METHOD OF MAKING CELLULOSE ORGANIC DERIVATIVE FILM AND SHEETING Filed Dec. 10, 1938 '5 Sheets-Sheet 2 mlllllllf'lll lll FIG.3.

VISCOSITY TEMPE RA TURE CHARLES R .FORDYCE WALKER E HUNTER, JR.

INVENTORS May 11, 1943. c, R. FORDYCE ETAL 2,319,052

HIGH SPEED METHOD OF M"KING CELLULOSE ORGANIC DERIVATIVE FILM AND SHEETING' Filed Dec. 10,1938 5 Sheets-Sheet 5 F IG' 4. FIG 5.

w m m M P m a 6 4 2 a 7 1! M L A B @0 w M 41/ 0C a w 5 m. P W M 0 :L 3P M M m2 5 m I O UB T 0 ibw kmxst N Z M M M M 8 6 4 .2 0 Wu 7 w w w a N 0 M M L 0V: m m m2 5 m FIG 6;

SOLUBlL/TY BOUNDARIES FOR CELLULOSE ACETATE CELLULOSE ACETATE PROPIONATE, AND ACETATE BUTYRATE IN PROPYLENE CHLORIDE, WITH ALCOHOLS A5 INDICATED, AT 20C.

Ebq EMIQI N CIMRLES R FORDYCE WALKER RHUNTER, JR.

INVENTORS 1943. c. R. FORDYCE ETAL 2,319,052

HIGH SPEED METHOD OF MAKING CELLULOSE ORGANIC DERIVATIVE FILM AND SHEETING Filed Dec. 10, 1938 Sheets-Sheet 4 Fla; 7. FIG.8.

20 g 4 ,8 7 ,8 Jon/515E 1 sows? g 7 l6 V /6 A /4 l4 V65 g V 6. 1 ,2 4, /0 E I V 4 I0 v %/N$0LUBLE 8 5 N 8 if, V d 6 i j INSOLUBZE 6 i v z 4 4 2 2 /o 20 3o 5o 60 70 /o 20 30 n n-BUTYI. ALCOHOL z ISO-BUTYL ALCOHOL FIG .9. 20 FIGJO. 20

A 18 A 18 fi SOLUBLE l6 SOLUBLE V g ,6

I y a 2 i /2 w 'Vmfl, log "/4 v I0 Ii f 4 i 8 i V INMLUBLE 6 R gl %/N50LUBZE 6 4 E 7 4 Z I Z Z /0 20 30 4o 50 6o 1o 20 30 40 5o 60 70 74 SEC-BUTYL ALCOHOL TERTIARY BUTYL ALCOHOL CHARLES R FORDYCE WALKER F. HUNTER, JR.

INVENTORS Patented May 1 l, 1943 I I 2,3 9,0 UNITED STATES PATENT OFFICE I HIGH SPEED METHOD OF MAKING CELLU- LOSE ORGANIC DERIVATIVE FILM AND SHEETING Charles R. Fordyce and Walker F. Hunter, Jr., Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application December 10, 1938, Serial No. 245,020

5 Claims.

This invention relates to a high speed method of making attenuated cellulose derivative products, such as film and sheeting, and more particularly to a method of making such products which is characterized by the use of cellulose organic acid ester compositions of hitherto unknown properties. 1

As is well known, cellulose derivative sheets or films are ordinarily produced by depositing a cellulose derivative solution or dope in the form of a film on the highly polished surface of a slowly rotating wheel or band, causing the film to set by evaporation of solvent, stripping the film and curing out residual solvent. The dope compositions heretofore employed for this purpose have been solutions which set or reach a solid or-semi-solid condition, permitting removal from Numerous attempts have been made to realize some improvement, until the advent of the present invention the ideal operation has never been attained.

As a further indication of the state of the art, it may be said that the broad phenomenon of gelation of certain types of cellulose derivative solutions under the influence of temperature change the forming surface only by gradual evaporation of solvent. With such dopes most of the solvent must be removed (leaving not much more than -25% of solvent, based upon the weight of the sheet) before satisfactory stripping of the film can be accomplished. This necessitates a relatively long period of preliminary curing on the wheel. Furthermore, the length of time required for proper setting is increased by the fact that. since such dopes remain fluid or semi-fluid until most of the solvent has evaporated '(and, therefore, must be supported on the wheel surface), evaporation of solvent can take place only from the outside surface of the deposited film. In addition, such dopes tend to skin over on the outside surface because of more rapid loss of solvent from the upper layers of film material and this further increases the setting time.

The advantages of bringing the film material into a solid or semi-solid condition as early in the film-forming operation as possible are apparent. Obviously, any reduction in the stripping time, that is, the time during which the film must remain on the Wheel before it can be properly stripped, directly increases production speed. Moreover, if the film can be removed from the wheel while still containing considerable solvent, more rapid curing can be attained. because under such conditions the film can be so handled and treated as to permit curing out of solvent from both surfaces simultaneously. tional advantage is that early solidification or colloidization results in a preferred micellar matlike structure with attendant improvement in the quality of the finished product. The ideal filmhas been observed from time to time by various workers in thecellulosic field. It has been recognized, for example, thatcertain organic liquids which are non-solvents for cellulose acetate and other cellulose organic acid esters at ordinary temperatures become solvents at elevated or moderately elevated temperatures and that if solutions are formed at the high temperatures and coated on a metal or other surface and cooled down, a tenaciously adhering lacquer coating results. It has also been recognized that by heat- "ing a suspension of cellulose acetate in ethylene dichloride (a cellulose acetate non-solvent at ordinary temperatures) to about 30-60" C., the

- cellulose acetate goes into solution to form a An addisults.

clear solution and when such a solution is coated on a surface, cooled and cured to remove thesolvent, a clear transparent film re- In other words, while a hot ethylene chloride solution of cellulose acetate will gel. upon coating or casting upon a film-forming surface, this phenomenon does not increase the speed of production of sheeting therefrom because such a film cannot be stripped and handled while containing any more solvent than the ordinary cellulose acetate dope wherein acetone and the like are solvents. In other words the gel so formed is not self supporting. Workers in this field have never gone much beyond a recognition of the phenomenon that certain dopes are capable of gelling and others are not. Until the present invention, no practical application of the phenomenon of gelation to film-forming op- .erations has ever been made.

forming operation would, therefore. be one in stripping time to a minimum) and curing solvent 1 from both surfaces of the film simultaneously;

ingor casting a dope on a film-forming surface,

characterized by the fact that the film may be removed or stripped from the surface while still containing a large proportion of solvent. A still further object is to provide a method of making such film or sheeting in which film formation takes place almostimmediately upon deposition of the dope. Another object is to provide a method of cellulose ester film or sheet formation in which the film can be removed from the forming or casting surface almost immediately after gelation while containing large proportions of solvent and is in such condition that residual solvent may readily be cured out of both surfaces simultaneously. Another object is to produce cellulose organic derivative sheeting having high tensile strength and flexibility and a low swell and shrink amplitude. Other objects will appear hereinafter.

These objects are accomplished by the following invention which, in its broader aspects, cornprises dissolving at elevated or moderately ele-- vated temperatures certain cellulose organic acid esters such as certain cellulose acetates, cellulose acetate propionates and cellulose acetate butyrates in a solvent consisting of propylene chloride and a mono-hydric aliphatic alcohol of 2-5 carbon atoms, whereby a solution or dope is obtained which is susceptible of gelation by rapid lowering of temperature to produce a sheet or film having such strength in the gel state that it may be stripped from the casting surface almost immediately after casting and while still containing nearly all or at least a large proportion of the original hot solvent. Specifically, the alcohols which we may use with satisfactory results are ethyl, n-propyl, iso-propy'l, n-butyl, iso-butyl, sec-butyl, 3butyl, n-amyl, iso-amyl, sec-amyl and 3amyl.

We have found that solutions of this character, the composition and preparation of which will be described in more detail hereinafter, possess certain unusual and unexpected characteristics which render them outstanding for the specific purposes of the instant invention. Among other things, 1) they are fluid at temperatures above 50 C.; (2) when allowed to cool to or below a critical temperature between 10-50 C. (depending upon the composition) they form entirely transparent gels which remain homogeneous throughout the gelling operation, such gelation occurring within approximately 20 C. of the fiowable solution point; (3) the gels when first formed do not adhere strongly to surfaces such as metal, glass, etc.; (4) the gels are sufficiently strong and resistant to deformation that they can be handled while still containing large quantities of solvent, i. e., an amount of solvent equal to or greater than the weight of the cellulose ester; (5) the nature or structure of the gels is such that they readily release their volatile solvents and the solvent can be driven on" without employing high temperatures.

Inasmuch as it is necessary only tocoat or cast the warm solution, cool, and strip almost immediately (due to the fact that the cold-setting or gelation effect produces at once a strong tough gel) an unusual and wholly unexpected increase in film-forming speed is attained. When one takes into account the fact that ordinary filmforming processes generally involve the use of dunes which require in some cases as much as fifteen or twenty minutes preliminary curing on the casting wheel or other surface before the material reaches a stage in which it can be successfully stripped, the tremendous increase in manufacturing speed made possible by the present method will be apparent.

In the following examples and description We have set forth several of the preferred embodiments of our invention, but they are included merely for purposes of illustration and not as a limitation thereof.

In the accompanying drawings:

Fig. 1 is a diagrammatic elevational sectional view of'a conventional type of device which may be employed for carrying out a typical filmforming operation in accordance with our invention,

r Fig. 2 is a chart showing graphically the various cellulose organic acid esters which may be employed within the teaching of our invention.

Fig; 3 is a graphical representation of the, viscosity changes which occur when certain typical compositions of our invention are cooled from their solution temperatures to or below their gelation temperatures or temperature ranges Referring first to Fig. 2 it is a triangular chart to identify the chemical composition of the cellulose esters under consideration. The composition in per cent acetyl is plotted along the line AB and the per cent higher acyl (such as propionyl or butyryl) is plotted along the line AC. The points ta." tp and "tb represent cellulose triacetate, tripropionate and tributyrate, respectively. The line connecting ta" and ."tb represents fully esterified mixed esters of acetic and butyric acid and the line connecting ta" and tp represents fullyesterified mixed esters of acetic and propionic acids. Hydrolyzed mixed esters fall within th eareas lying above the fully esterified products.

On this chart are outlined areas I, H, and III, representing, respectively, cellulose organic acid esters which are susceptible of forming solutions of the gelation type in mixtures of propylene chloride and alcohols in accordance with our invention, esters which are insoluble in such solvent mixtures either at room temperature or at elevated temperature, and esters which are so soluble at room temperature or slightly below as to be incapable of producing gelation type solutions.

In order to prepare a solution of a cellulose esterwithin the area of composition I which will exhibit the property of forming a gel upon allowing the warm solution to cool, it is desirable to employ a proportion of propylene chloride and alcohol which will be suitable for the particular composition of cellulose ester to be used. For this purposethere are outlined in Figures 4 to 14 the proportions of each individual alcohol with propylene chloride which will show the desired gelling characteristics with cellulose acetate and with the cellulose mixed esters of varying higher acyl content.

In each of the charts of Figs. 4 to 14 there are illustrated areas indicating the solubility of the specified cellulose organic acid esters. In each case the extreme left-hand area represents compositions in which the esters are insoluble in the indicated solvent combination even at elevated temperature; the area next from the left represents compositions in which the esters are so soluble at room temperature as to be unsusceptible of gelation in accordance with our invention; while the centralarea represents compontions in which the esters are susceptible of gelation when temperature of the solution is lowered to a. temperature within the range of Ill-50 C.; while the extreme right-hand area represents compositions which are also insoluble even at elevated temperatures and are therefore unsusceptible of gelation in accordance with our process.

It will be understood from these charts that the cellulose esters employed in accordance with our invention are cellulose acetates of approximately 39-42% acetyl, and "cellulose acetate propionates and cellulose acetate butyrates containing not over about 35% higher acyl and not less than about 39% total acyl, or more specifically,

those cellulose organic acid esters having the composition indicated by area I of Fig. 2. It should be noted however that the numerical ranges of acyl content just given are not exact, except as referred to area I of Fig. 2 since there is a small proportion of such esters which are inoperative in accordance with our invention, specifically, those esters falling within the areas II and III of Fig. 2. Therefore, when we refer herein and in the claims to cellulose acetate propionates and cellulose acetate butyrates containing about 35% higher acyl and not less than about 39% total acyl, we refer specifically to Referring to Fig. 1 of the drawings, numeral l designates a dope storage or supply tank provided with an inlet conduit 2 for admission of the previously prepared dope. The tank is provided with a removable cover 3 for permitting inspection of the contents and for other purposes and also provided with a heating coil 4 through which a flow of an appropriate heating fluid such as hot water or steam is maintained those esters which will fall within area I of Fig. 2.

Plasticizers may be used in varying quantities in the above compositions and have a minor effect upon the gelation behavior. Use of triphenyl phosphate in quantities as high as of, the weight of the cellulose ester does not produce any measurable change in gelation temperature,

stripping time, or other phases of the film-forming operation. Liquid plasticizers used in large quantities usually require a minor adjustment in solvent mixtures, such as a decrease in the quantity of more active. solvent by 540%.

We have referred to the viscosity characteristics of the various compositions adapted for use in our process, and it is accordingly desirable at this pointto describe the method by which vis-. I

cosity is measured. This is a modification of the widely used dropping ball method,'the procedure being as follows: A

The dope under examination is filtered and poured into a test tube having a depth of 150 mm., a diameter of 15 mm. and containing a steel ball in diameter weighing .4400 gram. The tube is filled to the brim with the dope under test and a cork stopper inserted with pressure enough to force air bubbles and excess dope past the cork. A small wire may be placed alongside the cork to facilitate the passage of air bubbles anddope past the stopper. The glass tube carries two scratches positioned exactly 10 cm. apart. a The dope-filled tube is then placed vertically in a constant temperature water bath with the stopper down. After the bath and tube have reached equilibrium temperature (usually within a period of one-half to one hour), the tube is quickly inverted and placed in a vertical glass cylinder. placed in the water bath. When the bottom of the steel ball reaches a position level with the first scratch, a stop watch is started and the time required for the bottom of the steel ball to reach a position level with the second scratch is measured. The viscosity is recorded as the time in seconds required for the ball to travel this 10 cm.

' distance between-the two scratches. 4

by means of thermostatically controlled valve 5. The flow of the heated fluid is so regulated as to maintain the dopein the tank I at a constant temperature.

Numeral 6 designates a feed conduit (which may be provided with lagging of an appropriate type for preventing heat losses as far as possible) through which the heated dope is passed to a standard form of dope hopper I, fiow of the dope being controlled by means of valve 8.

The dope hopper is provided with an adjustable gate member 9 for controlling the thickness of the dope stream which flows from the hopper. Adjustment of the gate member 9 may be by thumb screw Ill threaded through one wall of the hopper. The hopper is provided with a cover It to prevent solvent and heat losses and is also preferably supplied with external or internal heating means (not shown) for maintainingthe dope at a constant temperature.

Positioned below the hopper l is the coating or casting wheel l2 mounted in suitable bearings l3 and surrounded by air casing I4, the wheel being adapted to rotate in the direction indicated by the arrow. The wheel is provided with appropriate cooling means inot shown) whereby its film-forming surface is cooled to an appropriate temperature equal to orbelow the gelation temperature of the particular dope employed in a given film-forming operation. Casing I4 is provided with air inlet conduit l5 and outlet conduitjllifor conducting a current of heated air '*around the wheel counter-currently to the path of the film undergoing formation.

The wheel is driven by appropriate mechanism (not, shown) of such nature that any desired rotational speeds may be attained. Numeral l1 designates a' conventional stripping roll over" which the formed film passes on its way to the curing device, which comprises a plurality of air material passes on its way to the wind-up 55 located in the last air section 20. These rolls are driven, preferably by means of the so-called tendency drive which permits the film to travel through the air section in a substantially freely supported condition, this type of drive compensating for any longitudinal changes of dimension which may take place in the film material during the curing operation.

The numeral 56 designate-s a hinged door which gives access to the last air section 20 and through which rolls of the finished product may be removed from time to time.

A typical film-forming operation may be carried out as follows:

An appropriate dope composition, previously thoroughly mixed in another container at an appropriate temperature, is fed into the mixing tank I through the conduit 2. Care is taken to maintain the dope, prior to contact with the wheel surface, at a temperature well above its gelation point and in a readily fiowable condition. The warm dope passes by means of conduit 6 into dope hopper I from which it fiows onto the wheel in a stream, the thickness of which is regulated by appropriate adjustment of gate member 9 to give the desired eventual film thickness, for example, .005 inch.

As previously indicated, the wheel surface is.

maintained at a temperature equal to or below the gelation temperature or temperature range of the particular dope in question and the wheel is driven at such a peripheral speed as to give the desired speed of film formation. As the dope contacts the cold wheel surface gelation takes place almost immediately, and, at the expiration of a substantially insignificant period of time. the film material has reached a condition in which it may be removed from the wheel at the stripping roll i'l. Although it is not necessary to subject the film to any considerable amount of curing on the wheel, it is generally best to remove a certain amount of solvent from the gelled film material at this point in the process, To this and air is admitted to wheel casin l4 through conduit I and passes countercurrently around the outside surface of the film, th solvent-laden air being finally conveyed out of the apparatus through conduit Hi. The air temperature may be adjusted to or below room temperature or it may be heated to as high as approximately 40 C. or over, the particular temperature depending upon the composition of the dope in question, the wheel speed, and various other factors.

The nature of the dope being such that it sets almost immediately into a rigid gel upon contacting the cold wheel surface, the film may be readily stripped upon reachingstripping roll l1. At this point the film contains a substantial amount of solvent, the exact amount, of course, being dependent on wheel speed, temperature of the casing air and other factors. parent, when it is practical to operate the wheel at a sufficiently high speed, the film may be removed from the film-forming surface while still containing practically all of its original solvent. Under no circumstances is it necessary to bring the solvent content down to a point below that at which the weight of the solvent equals the weight of the cellulose organic acidester. Under ordinary circumstances the wheel is operated at such a speed that the filmcontains anywhere As will be ap- Our invention will beinore readily understood by reference to a number of specific examples illustrating preferred embodiments thereof.

Example 1.A solutionof 100 parts by weight almost immediately to a rigid gel under the infiuence of the lower temperature. The material was allowed to'remain in a current of air at approximately 20 C. for five minutes, whereupon it was stripped from the surface and cured to remove volatile solvent. The resulting clear, transparent film was .005 inch in thickness, and was found by test to be of superior tensile strength and flexibility to similar films cast by the customary evaporative method from acetone solution.

Example 2.A solution of ,100 parts by weight of a cellulose acetate propionate containing 30% acetyl and 14.5% propionyl content in 600. parts by weight of a solvent mixture composed of 53% by weight of propylene chloride and 47% 3amyl' alcohol'and containing 10% triphenyl phosphate, based on the weight of thecellulose ester, was prepared by mixing theingredients with continued stirring at 60 C. The solution was then filtered to remove incompletely dissolved particles and fed to the supply tank of a film-forming apparatus such as that illustrated in Fig. 1. The temperature of the dope in the tank was maintained at 60. C.

The dope was admitted to the hopperwhere its temperature was maintained at about 50 C.

The gate of the hopper was so adjusted as to feed a stream of the 'warm dope to the cold wheel surface in such an amount as to give an eventual film thickness of .005 inch; the wheel being maintained at aconstant temperature of about 25 C. The wheel was rotated at a speed such that the film remained on the film-forming surface for about six minutes'during which time a current of air having an inlet temperature of about 50 C. was passed through the space around the wheel in a direction counter-current to that of the movement of the film.

The warm dope, immediately upon comingin contact with the cold wheel surface, was transformed into a non-fiuid gel. After completing somewhat more than three-quarters of a revo- 'lution on the wheel, the film was stripped from from 50% to 80% of solvent at the time of stripping.

After stripping. the film is conducted into the first air section l8, where it is subjected to the action of a current of air heated, for example, to about 40-60" C. Solvent is removed progressively with travel of the film through the air section. The film upon emerging from the first air section passes immediately into the next air section where it is subjected to the action of air heated to a temperature of about 40-80 C. and finally into the air section 20-, where it is subjected to the action of air heated from about 85-95 C. By the time the film reaches the wind-up it has lostsubstantially all of its original solvent content and is then in suitable condition for useas photographic film support and many other purposes.

the film-forming surface and was thereafter carried through the, three air sections where it was subjected to the curing action of a current of moderately heated air. The air passing through'the first air section had an inlet temperature of about 50 C. providing an average temperature in the section of 45 C. The path and speed are such that the film in this sectiontook approximately 16 minutes to travel there through. The average temperature of the sec- -ond air section was C., and of the third 'The film at the point of stripping was foundto contain about 60% solvent under the par-. ticular' conditions of coating. The finished wa found to have high tensile strength, high flexibility, and a swell and shrink amplitude of less than .56%.

As further examples of mixtures of propylene chloride with various alcohols which may be empioyed-to dissolve a cellulose acetate propionate of 29.9% acetyl and 14.5% propionyl content for coating films under conditions similar to those of Example 1, the quantities of alcohol in the solvent mixture and the characteristics of the resulting solutions are given in the following table:

Further examples of the treatment of a cellulose butyrate of 31.1% acetyl and 16.0% butyryl content are given in the following table:

\ Casting Strip- Lx- 'lotal Gelhng h s rf ample Alcohol used Solvent Alco ol mull-L timalce Grams Percent C'. C. ltlin.

Ethyl 500 40 35 26 3. Iso-propyl... 500 40 35 27 2.0 N -propyl 500 40 35 28 2. 0 3-bllt}1.... 500 45 35 31 3.6 sec-butyl. 500 40 35 28 i. 1S0-bl1tyl 500 30 30 28 4. 0 N-hlltYl... 500 30 30 27 2.0 .i-amyl- 500 40 35 29 1. 5 sec-amyl. 500 30 30 27 2. 5 lso amyl 500 20 30 27 2. 5 N-arnyl 500 20 a0 27 2.0

As additional examples of the use of cellulose acetate of 40.4% acetyl content, the quantities Table of physical properties of cellulose acetate propionate films coated from i vent combinations composed, respectively, of 1 propylene chloride and iso-propyl alcohol, propylene chloride and tertiary butyl alcohol, and propylene chlorideand tertiary amyl alcohol, in the proper proportions as indicated by the composition charts of Figures 4-14, outstanding results are obtained. In other words, these particular solvent combinations constitute a subenus of our broad invention which is outstanding. Specifically, the compositions of Examples 6,15, and 26 above have been found to give particularly desirable results in the manufacture of photographic film support.

In this connection, it is important to note that the matter of permissible solvent content at stripping is one of the distinguishing features of our invention. Film or sheet material produced be attained. Our compositions, on the other hand, are of such nature that they may be satv isfactorily stripped from the film-forming surface while containing anywhere from to 80% solvent. It will thu be seen that the film or sheet material of the instant invention is of a fundamentally diflerent nature than similar products produced from the non-gelling types of dope of the prior art.

Sheet material obtained by following the procedure set forth above is found to be outstanding in certain physical properties as compared with sheets or films composed of the same cellulose ester but produced in accordance with the standard prior art methods, namely, by gradual evaporation of solvents from a deposited layer of the film-forming composition. As will be seen from the comparative data in the following table, the most outstanding advantage of our products are increased tensile strength, flexibility, and diminished dimensional swell and shrink of the film in alternately wet and dry condition. I

dz'fierent solvents (containing 10% tfiphenyl phosphate on the cellulose ester) train: 100% 100% dichloride acetone Qthylelw 3-a1'yl dichloride methanol 81 00h 01 A B o D Tensile strength 2 16. 4 16. 0 16. 5- 22. 7 Flexibility ...iolds. 7 8 14 63 Stretch per cent. 31 30. 4 32 47. 5 Swell and shrink amplitude; "110.--- l. 1 1. l 0.95 0. 56

of solvent and conditions of film formation are 0f the above film-forming compositions we have found .that when the above indicated cellulose acetates, cellulose acetate propionates and cellulose acetate butyrates are dissolved in sol- The above table illustrates the remarkabl improvement in physical properties of film produced in accordance with our invention as compared to films produced from the same cellulose ester by conventional evaporative methods of coating or casting. For example, it will be 'seen that the tensile strength of films A, B, and C, produced according to standard practice, i not above 16.5 kg s., whereas the tensile strength of our product (film D) is 22.7 kgs., an increase of about 27%. As to flexibility, the number of folds which films A, B, and C will withstand is only '7, 8, and 14, respectively, while the number which our film D will stand is 63, this representing a marked increase in flexibility for our product.

One of the most outstanding differences between films or sheets produced in accordance with our invention, and similar prior art products, is the fact that they have an extremely low swell and shrinkamplitude, that is, .the property of undergoing linear dimensional change in alternately wet and dry condition. As is well known, the swell and shrink characteristics of a photographic film, for example, are of great importance and the most useful films are those having the lowest swell and shrink amplitude. This is of particular importance in films which are to be used for X-ray, portrait, or aerial photography where sheets of appreciable size are employed. Obviously films of high swell and shrink characteristics tend toward internal unevenness which is due, either to buckling of the film in the center, or to curling of the edgesphenomena which are absent from films having a low swell and shrink amplitude and the ability to lie flat without curling. Other types of film which are used in long strips, such as rolls of Cine films, are difiicult to processsuch materials if of high swell and shrink characteristics, exhibiting appreciable shrinkage after removal from developing or washing solutions, at which time the films ar usually mounted on a drying rack. Under such conditions these films-tend to become severely tightened resulting in distortion of the film base and the photographic image carried thereby. It has been proposed to reduce the tendency of such films to swell and shrink by incorporating therein a fairly large amount of a water-repellent plasticizer. However, the use of such a plasticizer in amounts sufficient to reduce the swell and shrink tendency to any appreciable extent has a detrimental effect on the physical properties of the film, causing a loss in tensile strength and solved in solvents at room temperature, coated, for example, on a glass plate to the same thickness, set or solidified by evaporating the solvent in dry air at room temperature, and curing in an oven at elevated or moderately elevated temperature.

While we do not confine ourselves to any particular theory or explanation of the results obtained, it appears that both the facility and speed with which our new products may be removed from the film-forming surface and their specific physical properties, particularly high tensile an increase in stretch. Another alternative is to employ a mixed cellulose organic acid ester and introduce into such ester a relatively high proportion of higher acyl groups. This method, similarly to the introduction of a high proportion of plasticizer, is also unsatisfactory, since, when an appreciable reduction in swell and shrink is obtained, a definite loss in tensile strength occurs and the resulting film is too limp for satisfactory use.

It is one of the features of our invention that we are enabled to produce a film or sheet from a cellulose mixed organic acid ester of the various types, having good tensile strength anddurability and containing, for example, as little as 10% or less, based on the weight of the ester, of a plasticizer, and obtain material having an unexpectedly low swell and shrink amplitude ranging from about 4% to about .8%, in most cases less than .8%hitherto unattainable results. In fact, these same materials when coated by the prior art method give swell and shrink ampitudes from 20% to 100% greater than the values obtained by our method. In other words, for any given plasticizer content and a given ester, we are enabled to obtain a film having a markedly lower swell and shrink amplitude than that of a film produced from the same ester by the evapostrength and flexibility and extremely low swell and shrink amplitude, are due to the fact that they set to a non-fluid state before curing. It is possible that the low linear dimensional ch'ange taking place when such films are alternately wet and dry may be due to a change in thickness rather than to a change in the length of the film on absorption of moisture, the swell and shrink very probably being dependent upon the mechanism by which the film itself was formed.

In order that the above-mentioned swell and shrink amplitude figures may be fully understood, the test for measuring this property of film or sheet material is given in detail below.

Swell and shrink amplitude test A sample of film or sheeting is conditioned and measured both before and after processing in a constant humidity room at a relative humidity of 50%, or as close thereto as is possible, and at a dry bulb thermometer reading of F. For photographic film support of cine positive thickness (.0055 inch) or less, the time of conditioning before processing should not be less than 1%, hours; after processing not less than 21/ hours. Film support of X-ray thickness (.008-009 inch) should be conditioned at least 2 hours before processing and 3-5 hours after processing. Sheeting of thickness greater than .009 inch should be conditioned longer or until equilibrium is established. An emulsion coated film material should be conditioned for at least 2 hours both before and after processing.

Strips 15 inches long and 1 inches wide are cut from the film material. Usually two strips from each sample lengthwise of the film material and two strips widthwise are used for the test and two sets of perforations are made in each strip. These strip are perforated on a punch and die perforating machine, the holes being approximately 10 inches apart. Measurements from outside edge to outside edge of the perforation holes are taken. Thus a reading, if

immediately taken, should be zero on the gauge. The gauge employed is graduated in thousandths of an inch and, since the perforations are 10 inches apart, the percentage of dimensional change may be read directly from the gauge by merely moving the decimal point one place to the right.

The strip are conditioned at 50% relative humidity and measured. They are then tacked loosely on a wooden rack and placed in a constant temperature thermostatically controlled at 125 F. for 30 minutes, spacing them in and out a minute or so apart to allow time for measuring. Care is taken to measure as speedily as possible-after the removal from the water after giving them a quick wipe with a towel to remove surplus water as shrinkage takes place almost instantly. The sample is then placed in an oven at 125 F. for one hour, then taken out and measured. This cycle is repeated three times or until the difference between'the wet and dry readings becomes constant. The difierence between the last wet and dry'readings in percentage is the per cent swell and shrink amplitude. This test measures the permanent, characteris. tic tendency of the film material to swell and shrink under the influence of absorbed and desorbed moisture, the difference between the lengthwise and widthwise measurements repremaking operation many variations in the solution temperature, wheel temperature, wheel casing air temperature, curing temperature, wheel. speed, and many other details of the process may.

temperature may be in the neighborhood of 10 to C., or at least sufliciently low to bring the dope to, and preferably below its gelation temperature.

At this point it may be well to discuss gelation temperature. By this term we do not necessarily senting the amount of non-uniformity in the I will be more readily understood by reference to Fig. f the drawings which illustrates graphically the change in viscosity which our solutions undergo upon lowering the temperature. Curve A was plotted from viscosity determinations made at various temperatures upon a film-forming solution composed of 100 parts of a cellulose acetate propionate of 29.5% acetyl and 15% propionyl content in 500 parts of a solvent mixture of 52% propylene chloride and 48% tertiarybutyl alcohol. It will be noted that the curve rises gradually as the solution is cooled from 70 C. and that upon approaching the temperature range of to C., a very marked increase in viscosity takes place. Continued cooling below about 37 C. results in extreme viscosity and gelation with the production of a rigid non-fluid mass. By employing varying concentrations of the cellulose ester in solution, the character of the curve is found to change somewhat. A more concentrated solution of the same cellulose ester in the same solvent combination would givea curve of the type B, While a lower concentration of the cellulose ester would give curve C.

It will be apparent that many additions to and variations in the above-outlined procedure are possible within the scope of our invention. For example, one may increase the temperature at which gelation will occur for a given solvent combination by increasing the proportion of alcohol of the solvent composition. In other words, referring to Figures 4-14, fora given cellulose ester,

lower quantities of alcohol within the gelation range would tend to give solutions which gel 'at lower temperatures whil higher quantities of alcohol would bring about corresponding increases of gelation temperature.

It will be seen iromthe above examples that no hard and fast rules can be laid down as to the composition of our film-forming solutions for all purposes, since the composition of a given solution will be adjusted in accordance with the particular conditions of coating, stripping and curing which are to be employed. In general, itmay be said that for a practical process' a given composition should be, in accordance with our invention, such that the cellulose derivative in question goes into solutionat temperatures at or above C. and remains fluid above that temperature. It should also be such that upon cooling it experiences a rather sharp increase in viscosity within a comparatively narrow temperature range of about 20 C.

It will be apparent that ina practical filmrefer to an exact temperature, but rather toa maximum temperature below whichthe cooling solution or dope undergoes a marked and rather tures below about 40 C.

The temperature of the wheel casing air, that is, the temperature employed to effect initial curing may also vary, as may the temperatures employed for curing after stripping. It is one of the advantages of our invention, however, that I due to the peculiar character of our film-forming operation in order that the final product may.

have the desired physical properties. In fact, the sheet or film material produced in' accordance with our invention should be subjected to the least tension possible during curing. This will be particularly desirable in those cases in which the film, after stripping, contains a very high proportion of the original solvent content.

Although our process finds particular application in the manufacture of photographic film support, it is broadly applicable to the manufacture of other types of sheeting, particularly thin sheeting adapted for wrapping purposes.

Our process hasmany advantages over known film-making processes, but'the most outstanding advantage is the tremendous increase in speed of film formation obtainable thereby. While we have referred to stripping times of anywhere from a minute or two to five or six minutes, there is noactual-theoretical limit to the stripping time, short of zero. In other words, according to our process, film or sheeting may be stripped almost immediately after coating. Itwill be appreciated, however, that the actual speed of a given practical film-making operation will be considerably lower than that theoretically obtainable. The operation may be slowed down by.

the practical necessity o-r desirability of applyv ing various subbing or backing treatments to the film support during the manufacturing operation. As a general'proposition, it may be stated that thefilm-making speeds obtainable by our process are far beyond anything which has thus far. been obtained in the film-making industry.-

of our invention is the fact that, due to their peculiar composition and characteristics, satisfactory gelling of our film-forming compositions is quite independent of the thickness of the deposited layer, although the thicker the layer, the lower is the casting speed'due to the relatively lower heat transference of thick layers as compared to thin layers. We may, however, produce films or sheets anywhere from a few ten thousandths inch or less to almost any desired thickness. It will thus be seen that our process is adapted, not only for the manufacture of photographic film support and even much thinner types of sheeting, such as those employed for wrapping purposes, but also for the manufacture of sheets adapted for use in the fabrication of laminated glass, container stock, and many other products.

What we-claim' is: 1. A high speed gelation process of making sheeting suitable for photographic film base which comprises dissolving at a temperature above 50 C. a cellulose organic acid ester selected from the group consisting of cellulose acetates of 39-42% acetyl, cellulose acetate propionates and cellulose acetate butyrates containing not over about 35% higher acyl and not less than about 39% total acyl, said cellulose esters having the composition indicated by the area I of Fig. 2 of the drawings, in a liquid which is a.solvent for the said cellulose ester only at a temperature above 50 C. and in a weight of such liquid, greater than the weight of the cellulose ester dissolved, which will give a solution which at a temperature within the range of l-50 C. will- .residual solvent from the film.

2. A gelable composition comprising a cellulose organic acid ester selected from the group consisting of cellulose acetates of 39-42% acetyl, cellulose acetate propionates and cellulose acetate butyrates containing not over about 35% higher acyl and not less than about 39% total acyl, said cellulose esters having the composition indicated by the area I of Fig. 2 of the drawings, dissolved in a liquid which is a solvent for the cellulose ester only at a temperature above 50 C., said liquid being composed of about 65-50% by weight of propylene chloride and 35-50% by weight of ethyl alcohol, and said liquid being of a weight,

greater than the weight of the cellulose ester;

dissolved, which will give a solution which will form a clear, transparent, self-supporting gel at a temperature within the range of -50 C. which at that temperature is sufliciently strong and resistant to deformation to permit handli while containing more than 50% solvent.

3. A gelable composition comprising a cellulose organic acid ester selected from the group consisting of cellulose acetates of 39-42% acetyl, cellulose acetate propionates and cellulose acetate butyrates containing not over about 35% higher acyl and not less than about 39% total acyl, said cellulose esters having the composition indicated by the area I of Fig. 2 of the drawings, dissolved in a liquid which is a solvent for the cellulose ester only at a temperature above 50 C., said liquid being selected from the group consisting of mixtures of propylene chlo-. ride and iso-propyl alcohol, propylene chloride and tertiary butyl alcohol and propylene chloride and tertiary amyl alcohol, said solution being se lected from the group of solutions corresponding to the shaded areas of Figs. 6, 10 and 14, respectively, and said liquid being of a weight, greater than the weight of the cellulose ester dissolved, which will give a solution which will form a clear, transparent, self-supporting gel at a temperature within the range of 10-50 C. which at that temperature is sufliciently strong and resistant to deformation to permit handling while containing more than 50% solvent. I

4. A gelable composition comprising a cellulose acetate containing about 40 acetyl dissolved in a liquid which is a solvent for the cellulose ester only at a temperature above 50 C., said liquid being composed of about 70% by weight of propylene chloride and 30% by weight of isopropyl alcohol and said liquid being of a weight, greater than the weight of the cellulose ester dissolved, which will give a solution which will form a clear, transparent, self-supporting gel at a temperature within the range of 10-50 C.

which at that temperature is sufliciently strong I ing not over about 35% higher acyl and not less than about 39% total acyl, said cellulose esters having thecomposition indicated by the area I of Fig. 2 of the drawings, in a liquid which is a solvent for the said cellulose ester only at a temperature above 50 C. and in a weight of such liquid, greater than the weight of the cellulose ester dissolved, which will give a solution which at a temperature within the range of 10-50 C. will form a clear, transparent, selfsupporting gel and which liquid is selected from the group consisting of mixtures of propylene chloride and iso-propyl alcohol, propylene chloride and tertiary butyl alcohol and propylene chloride and tertiary amyl alcohol, said solution being selected from the group of solutions corresponding to the shaded areas of Figs. 6, 10 and 14, respectively, casting the solution from a supply thereof having a temperature above its gelation temperature in the form of a film at a temperature of l0-50 C. on a film-forming surface, stripping the film while containing at least 50% solvent and removing residual solvent from the film.

CHARLES R. FORDYCE. WALKER F.- HUNTER, J R.

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